Aromatase (estrogen sythetase) inhibitors are being synthetised and evaluated in vitro for enzyme inhibition. The ability of active compounds to cause regression of carcinogen (DMBA)-induced, estrogen-dependent mammary tumors in the rat is being studied. To further elucidate the mechanism by which the most effective aromatase inhibitor is bringing about tumor regression, we will determine: 1. whether tumor regression can be reversed by estradiol supplements, 2. what effect this compound has on blood levels of steroid and protein hormones, and 3. on estradiol-receptor levels in the tumor. We will also determine whether the most effective aromastase inhibitor has activity at the estradiol-receptor level and whether the compound will aromatize to estrogenic compounds. BIBLIOGRAPHIC REFERENCES: Studies on the Involvement of P-450 in Estrogen Biosynthesis. H.J. Brodie, D. Kupfer and A.M.H. Brodie. Endocrinology 96, A-112 (1975). Estrogen Biosynthesis Inhibitors as Anti-fertility and Anti-tumor Agents. A.M.H. Brodie, A.A. Shaikh and H.J. Brodie. Endocrinology 96, A-238 (1975).